Both numerical (Re = 40 - 12000) and experimental (Re = 12000, 22000, 31000) investigations are carried out to study flow control around a finite surface-mounted square cylinder. The suction and blowing are employed through a slot located on the upstream face of cylinder as an active flow control. The current study also investigates the Reynolds number (Re, based on the free stream velocity and cylinder width) and the aspect ratio (AR, cylinder height to width ratio, AR = 2, 4, 7 and AR = 7 for numerical and experimental works, respectively) effects on the flow structures for cases with and without flow control. The suction/blowing parameter, ?, is defined as the ratio of the mean velocity of the secondary flow to the free-stream velocity is changed as -4 ? ? ? 4, where the positive and negative ? representing blowing and suction, respectively. In this work, firstly, three-dimensional (3D) unsteady flow characteristics around a finite wall-mounted square cylinder is investigated for AR = 7 and Re = 40 - 250 via a direct numerical simulation. The results of the first part of this research show that the vortex shedding inception occurs within the range of 75 Re 85. The Re has a considerable effect on the mean wake topology and integral parameters. As such, the wake flow changes from a dipole to a quadrupole type, when the flow changes from steady to unsteady. A transition flow commence at a critical Re within Re =150 - 200, where the wake instabilities are intensified with increasing Re and the force signal oscillation alters from a sinusoidal to a chaotic type. Finally, the wake flow becomes turbulent for Re 200. In the second part of this dissertation, 3D unsteady flow around a finite-wall mounted square cylinder subjected to a steady secondary flow (blowing and suction) from the cylinder’s front face has also been studied with the aid of direct numerical simulations. All simulations of this part were carried out at Re = 250 to investigate the influence of ? (-4 ? ? ? 4) and AR (= 2, 4, 7) on the cylinder forces, vortex shedding frequency, and mean and instantaneous wake structures. The results show that, independent of AR, both blowing and suction have the potential to reduce the time-mean forces (e.g. more than 90% pressure drag reduction), fluctuating forces and vortex shedding frequency. Furthermore, a detailed investigation of vortex evolution revealed that the suction strategy considerably affects the shedding type and wake flow topology. Increasing the suction ratio changes the shedding from symmetric to asymmetric at AR = 4. Conversely, a planar jet attenuates vortical structures in the vicinity of the wall. Jet deflection was observed at ? = 2 and 4, which plays a key role in the success of the control approach and in the determination of the wake flow structure by changing the pressure distribution on the front face. Two critical AR values were determined where the flow and the mean wake structure changed from steady to unsteady (2 AR 3) and from dipole to quadrupole (AR = 4). In the last part of this study, large eddy simulation and wind tunnel experiments are utilized to investigate the turbulent flow characteristics over a finite square cylinder with AR = 7 at Re = 12000 and 22000. Moreover, the investigation of the flow control efficiency at high-Re and also the influence of the jet velocity profile on the performance of the control strategy are examined in this section. Key Words: Finite-wall mounted square cylinder; onset of vortex shedding; transition flow; turbulence flow; Dipole and quadrupole structures; Direct Numerical Simulation; Large Eddy Simulation; Anechoic wind tunnel; Hot-wire; Remote microphone; smoke visualization.